1. Field of the Invention
The present invention relates to processes for producing alkylated phenols and, more particularly, it relates to processes for alkylating phenols that have at least two replaceable hydrogen in the ortho-ortho or ortho-para positions.
2. Description of the Prior Art
Phenols that are alkylated in the ortho-ortho or ortho-para positions have numerous industrial uses. 2,6-xylenol, for example, is useful as the starting material for various polymers and 2,4-xylenol is the starting material for certain antioxidants. 2,3,6-trimethylphenol is the starting material for Vitamin E.
The method of alkylating phenols that have an open ortho- or para-position with an aldehyde and secondary amine to form an alkyl aminoalkylphenol that is then cleaved by hydrogenolysis to produce alkylated phenols is well known in the art. An example is U.S. Pat. No. 2,194,215 (Bruson et al.) which teaches the methylation of phenolic compounds by condensing the phenolic compounds with at least one molecular equivalent each of formaldehyde and a strongly basic, non-aromatic secondary amine to form phenolic tertiary amines. The phenolic tertiary amine obtained is then subjected to hydrogenolysis, whereby the secondary amine is reformed and a new methylated phenolic compound is produced. The hydrogenolysis is performed in the presence of the hydrogenation catalyst, copper chromite, at a temperature above 100.degree. C. and below 300.degree. C., and preferably between 150.degree. C. and 200.degree. C. By this process phenol can be converted into ortho-cresol, para-cresol, 2,4-xylenol, 2,6-xylenol or 2,4,6-trimethylphenol or mixtures thereof, depending upon whether one, two or three moles each of formaldehyde and a secondary amine are employed for the condensation.
The above described reaction will yield one product in good yield when the aldehyde and the amine are used in excess over the available ortho and para hydrogen as is shown by Bruson and McMullen [J. Am. Chem. Soc. 63, 270, (1941)]. However, when fewer than all the available ortho and para hydrogens are to be replaced, the reaction yields a mixture of mono-, di- and trialkylated phenols as well as positional isomers. Thus, Carlin and Landerl [J. Am. Chem Soc. 72, 2762 (1950)] made 2,6-xylenol from o-cresol rather than from phenol and still obtained less that 50 percent of the desired Mannich base, 2-methyl-6-dimethylaminomethylphenol. Similarly, Caldwell and Thompson [J. Am. Chem. Soc. 61, 756 (1939)] obtained the 2-aminomethylated 3,5xylenol from 3,5-xylenol in less than a 40 percent yield. These authors also opined [J. Am. Chem. Soc. 61, 2354 (1939)] that there was no clear-cut way of getting a good yield of the desired alkylated product. N. P. Greco (U.S. Pat. No. 4,215,229) found that predominantly monoalkylated phenols could be made from parent phenols containing more than one replaceable hydrogen. He also showed, however, that isomers are obtained in nearly equal amounts, and that some dialkylated derivatives are also obtained.
One way of overcoming some of the difficulties attending the prior art is the prior conversion of a phenol having more than one replaceable hydrogen atom to an alkyl derivative having only one replaceable atom. The desired alkyl group is then introduced with an aldehyde and an amine and the resultant Mannich base is converted to the final desired product by hydrogenolysis. Such a process is exemplified by the teachings of Gershanov et al. (U.S. Pat. No. 3,946,086). The above disclosure, however, does not teach how an alkylated phenol may be produced in a good yield and high purity from a phenolic compound having more available ortho or para positions than the number of alkyl groups to be introduced. It is, therefore, an object of the present invention to provide such a process.